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Cns Chapter 2


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Cns Chapter 2

  1. 1. Classical encryption techniques
  2. 2. Science of secret-overview
  3. 3. Conventional Encryption Principles <ul><li>An encryption scheme has five ingredients: </li></ul><ul><ul><li>Plaintext </li></ul></ul><ul><ul><li>Encryption algorithm </li></ul></ul><ul><ul><li>Secret Key </li></ul></ul><ul><ul><li>Ciphertext </li></ul></ul><ul><ul><li>Decryption algorithm </li></ul></ul><ul><li>Security depends on the secrecy of the key, not the secrecy of the algorithm </li></ul>
  4. 4. Conventional encryption requirements <ul><li>Strong algorithm </li></ul><ul><ul><li>For known plain-cipher text pairs, opponets should not be able to decrypt cipher text or the key </li></ul></ul><ul><li>Key secrecy </li></ul><ul><ul><li>Sender and receiver should receive </li></ul></ul><ul><ul><li>Store </li></ul></ul>
  5. 5. Conventional Encryption Principles
  6. 6. Cryptography <ul><li>Classified along three independent dimensions: </li></ul><ul><ul><li>The type of operations used for transforming plaintext to ciphertext </li></ul></ul><ul><ul><li>The number of keys used </li></ul></ul><ul><ul><ul><li>symmetric (single key) </li></ul></ul></ul><ul><ul><ul><li>asymmetric (two-keys, or public-key encryption) </li></ul></ul></ul><ul><ul><li>The way in which the plaintext is processed </li></ul></ul><ul><ul><ul><li>Block or stream </li></ul></ul></ul>
  7. 7. Attacks on conventional encryption system <ul><li>Cryptanalysis </li></ul><ul><ul><li>Attacks rely on nature of algorithm </li></ul></ul><ul><ul><li>Knowledge of plaintext characteristics </li></ul></ul><ul><li>Brute force attacks </li></ul><ul><ul><li>Attack by all combinations of key </li></ul></ul><ul><ul><li>On an average half of possible keys </li></ul></ul>
  8. 8. Average time required for exhaustive key search 2.15 milliseconds 2 32 = 4.3 x 10 9 32 5.9 x 10 30 years 2 168 = 3.7 x 10 50 168 5.4 x 10 18 years 2 128 = 3.4 x 10 38 128 10 hours 2 56 = 7.2 x 10 16 56 Time required at 10 6 Decryption/ µs Number of Alternative Keys Key Size (bits)
  9. 9. Types of cryptanalysis <ul><li>Cipher text only, algorithm known </li></ul><ul><ul><li>Brute force impractical for large keys </li></ul></ul><ul><ul><li>Must rely on type of plain text </li></ul></ul><ul><li>Known Plain and cipher text pairs </li></ul><ul><ul><li>Pattern of plain text </li></ul></ul><ul><ul><li>Deduce transformation and derive key </li></ul></ul><ul><li>Chosen plain text </li></ul><ul><ul><li>Introduce chosen patterns for encryption </li></ul></ul><ul><li>Chosen cipher text </li></ul><ul><li>Chosen text </li></ul>